Various cmake improvements

[Ri0n]

initial problem was missing protobuf-c on windows.

So added various way of building with protobuf-c suitable probably for every imaginable case.
Also added installation of cmake config files to make it super easy to use libomemo from other cmake projects.

For example I can build a bundled/static version with

cmake -DCMAKE_INSTALL_PREFIX=../omemo-static-install -DBUILD_WITH_PROTOBUF=bundled -DCMAKE_POSITION_INDEPENDENT_CODE=ON -DBUILD_SHARED_LIBS=OFF -DCMAKE_BUILD_TYPE=Debug ~/projects/psi/qomemo/

and then use it like

list(PREPEND CMAKE_PREFIX_PATH /home/rion/temp/omemo-static-install)
find_package(omemo-c)
target_link_libraries(mytarget omemo-c::omemo-c)

[Neustradamus]

@mar-v-in, @fiaxh: What do you think about this PR?